Hard disk drives (HDDs) have one or more disks on which ferromagnetic thin materials are deposited. Information recorded on the disks is generally organized in concentric tracks. As part of the manufacturing process permanent servo information is recorded on the disks that provides information to the system about the position of the heads when the disks are rotating during operation. The servo data on the disk provides several fundamental functions and is typically arranged in four distinct fields that are disposed in sequence along the direction of the tracks. First, it supplies a timing mark (known as the Servo Track Mark (STM) or Servo Address Mark (SAM)) which is used to synchronize data within the servo fields, and also provides timing information for write and read operations in the data portions of the disk. Second, the servo area supplies a 10-30 bit digital field, which provides a coarse track-ID (TID) number and additional information to identify the physical servo sector number. The TID is typically written in Gray code as the presence or absence of recorded dibits. During seek operations, when the head is moving across tracks, the head can typically only read a portion of the Gray-code in each TID. The Gray-code is constructed so that pieces of the TID, in effect, can be combined from adjacent tracks to give an approximate track location during a seek.
Finally, the servo field supplies a position error field, which provides the fractional-track Position Error Signal (PES). Auxiliary functions, such as amplitude measurement or repeatable run-out (RRO) fields are sometimes also used. During read or write operations the drive's servo control system uses the PES servo information recorded on the disk surface as feedback to maintain the head in a generally centered position over the target data track. The typical PES pattern is called a quad-burst pattern in which the bursts are identical sets of high frequency magnetic flux transitions. Unlike the track-ID (TID) field number, the PES bursts do not encode numerical information. In contrast to the TID, it is the position of the bursts that provide information on where the head is relative to the centerline of a tracks. The quad-burst pattern is repeated for each set of four tracks, so only local information is provided. Each servo wedge has four (A,B,C,D) sequential slots reserved for PES bursts. Each track has a centered PES burst in only one of the four slots. Each burst is centered on a selected track, but its width extends to the centerline of adjacent tracks. Thus, when the head is centered over a selected track, it will detect the strongest signal from a burst centered on the selected track, but it will also detect a weaker signal from bursts on the adjacent tracks. For example, when the head is centered over a track with a burst in the A-position, it might also detect a subsequent weak B-burst on the adjacent track on the right and then a weak D-burst from the adjacent track on the left. When the head passes over the PES pattern, the bursts that are within range generate an analog signal (waveform) that indicates the position of the head over the disk and is used as feedback to adjust the position of the head. Variations of the standard quad-burst pattern described above include use of two conventional, single frequency, quad burst servo patterns interspersed with dual frequency, dual burst servo patterns as described by Serrano, et al. in U.S. Pat. No. 6,078,445.
With relatively minor variants throughout the industry, the implementation of each of these servo functions has remained relatively unchanged since the advent of PRML recording technologies allowed digital signal processing techniques to be applied to the servo problem. Each of these functions typically consumes a relatively independent portion of the servo wedge in prior art servo systems.
The overhead on the disk to support these functions is a large factor in the drive's format efficiency. Typically, the servo fields can consume between 5% and 10% of the recording surface of the disk. As areal density gains in the magnetic and data signal processing components become harder and harder to achieve, the servo overhead becomes a more and more attractive target for reduction, and relief of necessary areal density targets to achieve particular HDD capacity points. The invention described herein provides a significant reduction in the servo overhead as compared to prior art systems.
U.S. Pat. No. 6,967,808 to Bandic, et al. describes a servo pattern having pseudo-random binary sequences for the servo information used to control the position of the recording head. A first pseudo-random binary sequence (PRBS) and a second PRBS identical to the first PRBS but shifted by a portion of the period of the first PRBS are located between the track boundaries in alternating tracks in a first region of the servo pattern and between the track centers in alternating tracks in a second region spaced along the track from the first region. A servo decoder of the invention has two correlators, one for each PRBS. Each correlator outputs a dipulse when its PRBS repeats. The difference in amplitude of the dipulses represents the head position signal. The dipulses also control the amplifier for the signal read back by the head and the timing of the track-ID (TID) detector. The AGC, STM and PES fields in the prior art are replaced by a pseudo-random binary sequence (PRBS) field. The TID field, which is not included in the PRBS, is encoded twice using non-return to zero (NRZ) encoding, which results in a smaller field and is more efficient than the prior art dibit encoding method used for Gray codes. The PRBS fields are also written using NRZ encoding. The first TID field is located between the two PRBS fields. In the preferred embodiment described the two PRBS sequences are formed by taking a PRBS and the same PRBS cyclically shifted by a portion of its period, preferably approximately one-half its period. This cyclic shift means that when the original sequence is input to the correlator matched to the shifted sequence there will be no output over a window with width equal to approximately half the sequence length, and vice versa. Over this range of lag values the two sequences are said to be orthogonal. One sequence (PRBS1) is referred to as the A/C sequence because it encodes both the A-burst and C-burst PES functions. The other sequence (PRBS2) is referred to as the B/D sequence because it encodes both the B-burst and D-burst PES functions.
Related prior art includes U.S. Pat. No. 7,193,800 to Coker et al. which describes the use of particular pseudo-noise (PN) or pseudo-random sequence fields for the purpose of PES and rudimentary TID detection. The AGC, STM, TID, and PES fields in the prior art are replaced by a pair of pseudo-random binary sequence (PRBS) fields. The two PRBS fields in a servo track are identical, but the PRBS fields in adjacent tracks are different. One set of alternating tracks uses a leading pseudo-random binary sequence (PRBS), which is a pseudo-noise sequence with good autocorrelation properties, and a following PRBS that is cyclically shifted from the leading PRBS. A second set of alternating tracks interleaved with the first set also has a leading PRBS and a following PRBS that is cyclically shifted from the leading PRBS, but the leading PRBS in each of the tracks in the second set is offset along-the-track from the leading PRBS in the tracks of the first set. The head positioning control system uses the leading PRBS to generate a servo timing mark (STM), the cyclic shift to generate track-ID (TID), and the following PRBS from adjacent tracks to generate the head position error signal (PES). The first PRBS field in alternate tracks (e.g., the odd servo tracks) are offset from the first PRBS field in the other tracks (i.e., the even servo tracks). The first or leading PRBS field on a servo track serves as the STM. The STM provides a reference for windowing the second PRBS field which is used to encode both the TID and the PES. The TID is encoded in the circumferential phase relationship due to a cyclic shift between the first PRBS field and the second PRBS field. The cyclic shift between the leading and following PRBS increases by a fixed increment with each track in the radial direction so that the length of the cyclic shift between the leading and following PRBS in each track represents the TID. The PES is derived from the relative contributions of the second or following PRBS field from the different servo tracks as the read head crosses adjacent servo tracks in the radial direction. The difference in amplitude of the dipulses from detection of the following PRBS in two adjacent tracks represents the PES sent to the disk drive actuator to maintain the head on track.
Published U.S. patent application 20090168227 by Blaum, et al. describes a method of distributed track-ID in which first and second portions of a track-ID are physically separated in a disk sector. Each of the portions of the track-ID is encoded using a Gray code.
Theoretical concepts from wireless communication techniques, such as Code Division Multiple Access (CDMA) can also be drawn upon for application to HDDs, but the problems in HDD imposes quite different constraints and require substantially different sequence sets. CDMA is a spread spectrum technology, which allows variable data streams for multiple users to co-exist at the same time in a given frequency band. CDMA assigns unique codes to each user's data stream to differentiate it from other streams. The sequence types used in CDMA and other advanced wireless communication techniques employ orthogonal sequence sets which maintain their orthogonality under cyclic extension and nonzero relative lag times. These sets are variously known as loosely-synchronous sets or zero correlation zone sets in the literature.